p-AlGaN Layer Research Articles

Study of Blue InGaN/GaN Light-Emitting Diodes With n-AlGaN Layer as Space Layer and Without the P-AlGaN Electron Blocking Layer

In this paper, we study the characteristics of blue InGaN light-emitting diodes (LEDs) with n-AlGaN electron blocking layer (EBL) and n-AlGaN space layer (SL). The output power, internal quantum efficiency (IQE), energy band diagrams, carrier concentrations, radiative recombination and spontaneous emission rate are investigated by advanced physical model of semiconductor device (APSYS). The simulation results indicate that the LED with n-AlGaN SL shows an improvement of output power and radiative recombination than that of the LED with n-AlGaN EBL. The efficiency droop has been alleviated due to the better carrier injection and the enhancement of the overlap for the electrons and holes wave-function.

Analysis of efficiency droop in 280-nm AlGaN multiple-quantum-well light-emitting diodes based on carrier rate equation

The efficiency droop in 280-nm AlGaN multiple-quantum-well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) is analyzed using the carrier rate equation. It is shown that the internal quantum efficiency (ηIQE), injection efficiency (ηinj), light-extraction efficiency (ηLEE), Shockley–Read–Hall recombination coefficient (A), and Auger coefficient (C) can be determined by the carrier rate equation using the theoretical radiative recombination coefficient (B), experimentally measured wavelength spectrum, and external quantum efficiency (ηEQE). The results show that the carrier spillover from the MQWs to the p-AlGaN layer is the main cause of the efficiency droop.

Significant Performance Improvement in AlGaN Solar-Blind Avalanche Photodiodes by Exploiting the Built-In Polarization Electric Field

We present improved AlGaN solar-blind avalanche photodiodes (APDs) with a separate absorption and multiplication (SAM) structure by introducing a polarization electric field with the same direction as reverse bias field in the multiplication region. This polarization electric field can be realized by reducing the Al composition of the p-AlGaN layer in a conventional p-i-n-i-n SAM-APD structure. After employing a reduced-Al-composition p-AlGaN instead of the commonly used p-AlGaN, the polarization enhanced APD exhibits a markedly lower avalanche breakdown voltage and a near two times higher avalanche gain up to 2.1 × 10 4 compared with its conventional counterpart. In addition, X-ray diffraction and transmission electron microscopy results show that a moderate reduction of Al composition in the p-AlGaN layer does not degrade the crystalline quality of the polarization enhanced APD structure resulted from lattice mismatch, which guarantees the polarization enhanced effect.

Effect of O Impurity on the Properties of InGaN/GaN Multiple Quantum Well and Light Emitting Diode Structures

ABSTRACT InGaN/GaN light emitting diode (LED) structures with different GaN barrier growth temperatures have been grown by metalorganic chemical vapor deposition (MOCVD). Atomic Force Microscopy (AFM)-Conductive AFM (CAFM) analysis has been performed to study the submicron scale electrical properties of the LED structures. The effect of high O impurity concentration, introduced during the low temperature barrier layer growth, on the MQW LED structure has been explored. It is observed that (VGa-ON)2- point defects formed in the MQW layer serve as deep-level traps and lead to defect-assisted tunneling. Increasing barrier growth temperature decreases O impurity incorporation and thus improves the device performance. INTRODUCTION InGaN/GaN MQW structures grown by MOCVD have been widely studied in the solid state lighting field for the past few decades. The role of introduced impurities and point defects formed during the epitaxial growth process and their effects on the optical properties of MQW structures have drawn a great deal of attention. Several previous investigations have been done on the effects of GaN barrier growth temperature on the characteristics of InGaN/GaN MQWs [1-3], in which the InGaN quantum wells were grown at low temperature from 700-760oC, while the growth temperature of the GaN barriers was varied over the range of 700-950oC. It was suggested that GaN barriers grown at temperatures below 800oC leads to the generation defects and In inclusions and higher barrier growth temperature up to 910oC improves the crystal quality of GaN barrier by suppressing the formation of deep level related defects. In the present work, the electrical properties of InGaN/GaN LED structure with same barrier growth temperature as the well temperature is studied. The effects of induced O impurity are discussed.EXPERIMENT The InGaN/GaN MQW and LED samples were grown on (0001) sapphire substrates with low temperature GaN buffer layers using Veeco K465i MOCVD reactor. 2.2µm undoped GaN layer and 2.7µm n-GaN layer were grown at 1085oC. 4 pairs of In0.14Ga0.86N/GaN (2.6 nm/10 nm) quantum wells were grown at 760oC for LED structure Al. 6 pairs of such quantum wells were grown at 760oC and 860oC for the InGaN wells and GaN barriers, respectively, for LED structure B1. 20nm p-AlGaN layer and 95nm p-GaN layer were grown at 980oC on top of the MQW structures to form the LED structures. AFM analysis was done on a Bruker Icon system and by using TESPA probe. CAFM module is mounted to the AFM system for conductivity studies of the sample. Extended-tunneling AFM tip holder and PFTUNA probes are used for AFM-CAFM analysis.EXPERIMENT RESULTS An AFM-CAFM study on LED structure with various reverse biases has been done on A1 and shown in Figure 2. Notice that in Figure (b) and (c), the color bar is from 450nA to 300nA, while 450nA to -400nA in Figure (d). High density of spots of early breakdown has been observed when the reverse bias increases. The high reverse current at those spots may owe to the defect-assisted tunneling by deep-level traps located in the MQW active layer [4]. This is consistent with the high O concentration and high point defects density in the MQW layer reported in Ref. [3]. These point defects cause leakage and deteriorate the device performance [5].CONCLUSION In summary, the InGaN/GaN MQW LED structure with 760oC quantum barrier growths has been investigated by AFM-CAFM system to study the submicron electrical properties of the sample. High density of spots of early breakdown caused by defect-assisted tunneling by (VGa-ON)2- complex point defects located in the MQW layer. These point defects cause leakage and deteriorate the device performance.

Analysis of the abnormal voltage-current behaviors on localized carriers of InGaN/GaN multiple quantum well from electron blocking layer

The effect of an electron blocking layer (EBL) on the V — I curves for GaN/InGaN multiple quantum wells is investigated. For the first time, we found that the curves intersected at 3.012 V, and we investigated the reason for the intersection. The forward voltage in LEDs with a p-AlGaN EBL is larger than it is without the p-AlGaN EBL at low injection currents because the Mg-doping efficiency for the p-GaN layer is higher than that for the p-AlGaN layer. However, the forward voltage in LEDs with a p-AlGaN EBL is smaller than it is without the p-AlGaN EBL at high injection currents because the carriers overflow from the active layer when the injection current increases in LEDs without a p-AlGaN EBL, in case of LEDs with a p-AlGaN EBL, the carriers are blocked by the EBL.

The Advantages of AlGaN-Based UV-LEDs Inserted With a p-AlGaN Layer Between the EBL and Active Region

The advantages of ultraviolet light-emitting diodes (LEDs) inserted with a p-AlGaN layer, whose Al mole composition is less than that of the last barrier, between the last barrier and the electron blocking layer have been investigated by using the Crosslight APSYS programs. The results show that the output power and the internal quantum efficiency of the proposed LEDs are improved. Furthermore, the efficiency droop is also mitigated effectively. Based on the analysis of electrical and optical characteristics, these improvements are mainly attributed to the relatively higher effective barrier height against the escape of electrons and an increased hole concentration in the quantum wells by inserting a hole reservoir near the active region. In addition, the optimized Al mole composition of this inserted layer has been also studied in detail, and the optimized Al mole composition has been achieved.

Exploitation of Polarization in Back-Illuminated AlGaN Avalanche Photodiodes

To improve the performances of separate absorption and multiplication AlGaN avalanche photodiodes (APDs), a polarization field and a polarization doping effect are introduced into the APDs by adjusting the Al composition of the p-AlGaN layer. The calculated results show that the polarization-induced electric field, which has the same direction as the reverse bias in the multiplication layer, can significantly lower the avalanche breakdown voltage. Further, the maximum multiplication gain increases pronouncedly due to the polarization-assisted enhancement of the ionization electric field at the point of maximum gain. In addition, the composition graded p-AlGaN layer can also improve device performances through the polarization doping effect. The employments of the polarization field and polarization doping can increase the maximum multiplication gain by as much as 225%.

Influence of Interference on Extraction Efficiency of Ultraviolet Vertical Light-Emitting Diodes

We report on enhanced efficiency of ultraviolet vertical light-emitting diodes (VLEDs) with interference between the reflective mirror and the multiple quantum well. The dimensions of the cavity are fixed at 30 nm for the p-AlGaN layer, while various thicknesses of p-GaN from 60 nm to 140 nm were used. The light output power of the VLED in constructive compared with destructive interference condition increased by 23.9% at 350 mA. These improvements could be attributed to the predominant constructive interference of vertical radiation due to an optical cavity with optimal p-GaN thickness.

Formation of AlGaN and GaN epitaxial layer with high p-carrier concentration by pulse supply of source gases

A drastic increase in the p-type carrier concentration and decrease in the activation energy of an acceptor are achieved by using a method of pulse supply of the source gases for AlGaN and GaN. This method offers a new way of producing a low-resistance p-AlGaN and p-GaN epitaxial layer.

Influence of polarization effects on photoelectric response of AlGaN/GaN heterojunction p-i-n photodetectors

Based on the simulation of the polarization effect by the sheet charge layer approximately, the energy band structures and electric field distributions of AlGaN/GaN heterostructures with different doping concentrations of p-AlGaN and polarization effects are calculated by self-consistenly solving the Poisson-Schrödinger equations. The corresponding photoelectric response is calculated and discussed by solving the carriers continuity equation. The results show the interaction between the doping concentration and the polarization effect has an important influence on the performance of the p-i-n detector. Specially, under the condition of complete polarization, a high doping concentration in the p-AlGaN layer will result in a narrow depletion region in p-AlGaN layer and the i-GaN layer will be depleted easily, which corresponds to a low photocurrent. Similarly, a strong polarization will result in a wide depletion region in p-AlGaN and high photocurrent for the same doping concentration in p-AlGaN layer. Finally, the effect of temperature on the performance of the detector is calculated and analyzed. It is concluded that AlGaN/GaN heterostructure p-i-n ultraviolet detector can be used in the high temperature environment.

Emission Efficiency Dependence on the p-GaN Thickness in a High-Indium InGaN/GaN Quantum-Well Light-Emitting Diode

The dependencies of quantum-well (QW) internal quantum efficiency (IQE) and device behaviors on the p-layer thickness in a high-indium InGaN/GaN QW light-emitting diode (LED) are demonstrated. During the high-temperature growths of the p-AlGaN and p-GaN layers, the QWs are thermally annealed to increase their IQEs and blue-shift the emission with increasing p-layer thickness. Meanwhile, the quantum-confined Stark effect is enhanced with increasing p-layer thickness to decrease the IQEs and red-shift the emission. Based on the counteraction between the two effects, the maximum IQE and the shortest emission wavelength are observed in a sample with an optimized p-layer thickness, which includes a p-AlGaN layer of 20 nm and a p-GaN layer of 60 nm in thickness under our growth conditions. The fabricated LEDs of different p-GaN thicknesses show the similar variation trends in emission efficiency and wavelength.

Ultra-violet light-emitting diodes with quasi acceptor-free AlGaN polarization doping

The development and application of nitride-based light-emitting diodes (LEDs) is handicapped by the low hole conductivity of Mg-doped layers. Mg-doping becomes increasingly difficult with higher Al-content of the p-AlGaN layers as required for ultra-violet (UV) light emission. Polarization-induced hole doping of graded AlGaN was recently demonstrated as an alternative doping method. Using advanced numerical device simulation, this paper investigates the impact of polarization-doping on the internal device physics of UV-LEDs and compares the conventional Ga-face growth to the novel N-face growth direction. Various LED design options are explored to maximize the internal quantum efficiency.

Optimized growth of p-type AlGaN electron blocking layer in the GaN-based LED

In the High-power InGaN/GaN-based LED structures, p-AlGaN layer plays a role as electron blocking layer. In this paper, GaN/InGaN-based LED have been grown on sapphire by metal organic chemical vapor deposition (MOCVD), and the p-type doping mechanism and structural optimization of AlGaN layer were studied. The ways to change AlGaN components have been discussed. We found that the growth temperature, growth pressure and flow TMAl (mole ratio) have strong effect on the Al components through different mechanisms. In the AlGaN electron blocking layer, the Al composition is between 10%—30% and the electron could be well limited to the quantum well region, maintaining a high quality crystal material. The p-type doping efficiency of AlGaN layer is low, and there is a magnesium droop problem due to lack of hole injection. A new growth method is suggestece to solve the problem. Grown under optimal conditions, the p-type AlGaN inserted in a LED structure greatly improves the output optical power of LED device.

Enhanced light output of InGaN LEDs with a roughened p-GaN surface using different TMGa flow rates in p-AlGaN layer

We have demonstrated the enhancement of light output of InGaN-based blue light-emitting diodes (LEDs) using different trimethylgallium (TMGa) flow rates in the growth of p-AlGaN epilayer to facilitate a rougher p-GaN surface. It is found that higher output power can be achieved from the LEDs with rougher surface morphologies when the TMGa flow rate ( R TMGa) is increased up to 60 sccm during p-Al 0.05Ga 0.95N epilayer growth. Such a rough surface obtained at higher R TMGa is attributed to the fact that the vertical growth rate is faster than the lateral growth rate, thus, leading to the facet of crystal growth focuses mainly in the vertical direction. The output power of devices biased at 20 mA is 15.4, 15.9, 17.5, and 18.9 mW for TMGa flow rates of 10, 20, 40, and 60 sccm, respectively.

Comparison of electrical properties and deep traps in p-AlxGa1−xN grown by molecular beam epitaxy and metal organic chemical vapor deposition

The electrical properties, admittance spectra, microcathodoluminescence, and deep trap spectra of p-AlGaN films with an Al mole fraction up to 45% grown by both metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were compared. The ionization energy of Mg increases from 0.15 to 0.17 eV in p-GaN to 0.3 eV in 45% Al p-AlGaN. In p-GaN films grown by MBE and MOCVD and in MOCVD grown p-AlGaN, we observed additional acceptors with a concentration an order lower than that of Mg acceptors, with a higher hole capture cross section and an ionization energy close to that of Mg. For some of the MBE grown p-AlGaN, we also detected the presence of additional acceptor centers, but in that case the centers were located near the p-AlGaN layer interface with the semi-insulating AlGaN buffer and showed activation energies considerably lower than those of Mg.

Formation Characteristics of Inversion Domains in P-GaN and P-AlGaN Layers By Using TEM Observation

The formation characteristics of the inversion domains in p-GaN and p-AlGaN layers were investigated by using transmission electron microscopy (TEM). The inversion domains induced by Mg-doped p-type GaN (p-GaN) and p-AlGaN layers were formed on a sapphire substrate by using a metalorganic chemical vapor deposition (MOVCD) method. The induced inversion domains were analyzed by using high-resolution TEM (HRTEM) and convergent beam electron di raction (CBED) and the residual strain distribution was evaluated by using a strain mapping simulation. The results revealed that pyramidal defects of various sizes were formed at the p-GaN/p-AlGaN interface. Also, a large number of inversion domains were observed to be initiated exclusively on (0002) plane of the pyramidal defects, which were above 10 nm toward the [000 1] direction, resulting in a rough, island-shaped p-GaN surface due to the inhibition of GaN growth by the inversion domains. In this study, the formation characteristics of inversion domains were correlated with the in uence of structural defects existing at the p-GaN/p-AlGaN interface.

Effects of the thickness of capping layers on electrical properties of Ni ohmic contacts on p-AlGaN and p-GaN using an ohmic recessed technique

In this study, the effects of the thickness of p-GaN (p-InGaN) capping layers grown on top of p-AlGaN (p-GaN) on electrical properties of Ni ohmic contacts on p-AlGaN (p-GaN) were investigated. The experiments and simulations indicated that the thicker p-GaN (p-InGaN) capping layer grown on p-AlGaN (p-GaN) led to the formation of the higher concentration of a two-dimensional hole gas (2DHG) at the interfaces. Consequently, we deduce that holes can be easily injected into the p-AlGaN (p-GaN) layer through recessed channels and a 2DHG channel, which results in the formation of the low contact resistivity. This provides a rational guideline for the development of new processing methodologies to enhance nitride semiconductor-based optoelectronic devices.

Inversion domain boundaries and phase separation in p-AlGaN layers with high Al contents

The effect of Mg incorporation and Al composition on the formation of inversion domain boundaries (IDBs) and self-assembled superlattices in Mg-doped p-AlxGa1−xN epilayers grown on undoped Al0.3Ga0.7N was investigated by transmission electron microscopy. The formation, initiating position, and shape of the faceted IDBs strongly depended on the Mg and Al contents. Horizontally well-aligned self-assembled superlattices with large composition modulation by phase separation were also observed in the Mg-doped p-AlGaN before the formation of the faceted IDBs. The periodicity of the modulation obtained from the diffused streaks is ∼40Å.

Enhanced vertical transport in p-type AlGaN∕GaN superlattices

We report on p-type AlGaN∕GaN superlattice designs with significantly improved vertical and lateral electrical conductivities (σV and σL). Composition-graded p-AlGaN layers produce a polarization charge distribution, which together with an appropriate Mg doping in the structure leads to more than an eightfold reduction of barrier height and a ∼50% increase in the sheet hole density in the p-GaN wells compared to typical modulation-doped superlattice structures. Using the optimized structure, more than 13 orders of magnitude and 35 times improvement is shown for σV, compared to typical superlattice and σL, compared to bulk p-GaN, respectively. Both σV and σL are found to improve significantly at higher temperatures in the optimized structure.

AlGaN-based ultraviolet light-emitting diodes grown on AlN epilayers

AlGaN-based deep-ultraviolet light-emitting diode (LED) structures, which radiate light at 305 and 290nm, have been grown on sapphire substrates using an AlN epilayer template. The fabricated devices have a circular geometry to enhance current spreading and light extraction. Circular UV LEDs of different sizes have been characterized. It was found that smaller disk LEDs had higher saturation optical power densities but lower optical powers than the larger devices. This trade-off between power and power density is a result of a compromise between electrical and thermal resistance, as well as the current crowding effect (which is due to the low electrical conductivity of high aluminum composition n- and p-AlGaN layers). Disk UV LEDs should thus have a moderate size to best utilize both total optical power and power density. For 0.85mm×0.85mm interdigitated LEDs, a saturation optical power of 2.9mW (1.8mW) at 305nm (290nm) was also obtained under dc operation.